Friction-modified wedge stemming plugs

ABSTRACT

A stemming plug ( 10 ) for stemming a blast hole in a mine, the plug ( 10 ) comprising an active wedge-shaped member ( 12 ) having a sloping face received in sliding relationship with a matching face of a passive wedge-shaped member ( 14 ). In use, the active wedge-shaped member ( 12 ) is positioned nearest to an explosive material in the blast hole and the passive wedge-shaped member ( 14 ) is positioned further from the explosive material in the blast hole. The active wedge-shaped member ( 12 ) has a friction reducing material provided on at least part of its surface to reduce the sliding resistance of the active wedge-shaped member ( 12 ) relative to the passive wedge-shaped member ( 14 ); whereby, in use, when a shockwave from initiation of the explosive material in the blast hole encounters the active wedge-shaped member ( 12 ) it acts as a piston, sliding on the passive wedge-shaped member ( 14 ) so that both wedge-shaped members exert diametrically opposed forces against a wall of the blast hole and are locked in place.

FIELD OF THE INVENTION

The present invention relates to mining, and more specifically to wedge stemming plugs for blocking off mining blast holes. The invention has particular application for stemming underground, upward sloping blast holes; however the product may also be applied to surface mining drill holes.

BACKGROUND TO THE INVENTION

“Stemming” describes both the inert material, and the act of placing inert material into a blast hole to contain the blast gases as much as possible on detonation. Stemming relies on friction, cohesion, or bridging of the stemming material to prevent rifling out of blast holes. The more the explosives are constrained, the greater the work they are able to do. Explosives follow the path of least resistance, and without stemming material large amounts of energy are lost through rifling out the open collar of the drill hole. With effective stemming in place, this removes that least resistance path and causes more energy to be used breaking the rock adjacent to the blast hole.

Stemming need not be airtight. In fact, in some circumstances it is advantageous not to be airtight, for instance when blasting with emulsion explosives. Emulsion explosives need to degas, and in doing this they will expand provided there is no resistance to the gas escaping by pressure build-up resulting from a sealed hole.

In underground metalliferous mining, blasting often occurs using overhead, upward sloping blast holes. In these circumstances, it is difficult to stem the blast holes because of their angle of inclination. Co-pending International Application No PCT/AU2013/000489 describes a method of using wedges to stem a blast hole. One embodiment of a stemming plug with wedges described in PCT AU2013/000489 does not include any dry grout material at all in the plug. However this plug may be limited to stemming only with the amount of force one applies during installation. The friction on both wedges in this configuration is equal, and this stemming plug may or may not wedge itself in more securely under the dynamic loading of blast initiation.

U.S. 2008/0047455 describes a rock breaking cartridge which comprises a tubular body that incorporates a stemming device therein. Upon activation, the stemming device expands a portion of the tubular body in a radial sense. A preferred embodiment of the stemming device in U.S. 2008/0047455 includes opposing wedges components made from wood or a suitable plastics material. The wedge components have mating sloping faces, so that when activated, (e.g. by a hammer blow applied to an end of one of the wedge components) the sloping faces ride over each other to radially expand the cartridge. The cartridge is thereby locked in position, with the expanded portion of the tubular body frictionally engaged with the wall of the hole.

U.S. 2010/0276984 similarly discloses a self-stemming cartridge comprising a cylindrical casing containing an accelerant and a stemming mechanism. The stemming device in at least one aspect consists of a dowel rod cut into separate dowels, one of which has opposing angled flat sides and the others forming wedges. The cylindrical casing may have a high friction external surface, such a sand paper.

The poor performance of most commercially available stemming devices at present, leads most mines to not stem upholes at all. This results in higher explosive use (and therefore cost), poor blast fragmentation, less effectiveness of the explosive charge, and greater damage to surrounding areas of the mine than would be the case with suitable stemming.

The present invention was developed with a view to providing a wedge stemming plug and a method of using wedges to stem an overhead blast hole, wherein the wedges will provide improved effectiveness compared to the prior art devices noted above. Although the invention is particularly useful for stemming underground, upward sloping blast holes, it is also of use in blocking drill holes for other purposes. For example, where drill rods are broken and stuck in an upward sloping overhead drill hole, but with some risk that they may dislodge of their own accord and fall out unexpectedly. In this instance the drill rods need to be anchored in the drill hole so that they can't inadvertently fall out when personnel are working below. The wedge-shaped members described can be used as a safety barrier to block the drill rods in a hole in these circumstances.

References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a stemming plug for stemming a blast hole in a mine, the plug comprising:

-   -   an active wedge-shaped member having a sloping face received in         sliding relationship with a matching face of a passive         wedge-shaped member wherein, in use, the active wedge-shaped         member is positioned nearest to an explosive material in the         blast hole and the passive wedge-shaped member is positioned         further from the explosive material in the blast hole; and,     -   the active wedge-shaped member having a friction reducing         material provided on at least part of its surface to reduce the         sliding resistance of the active wedge-shaped member relative to         the passive wedge-shaped member;     -   whereby, in use, when a shockwave from initiation of the         explosive material in the blast hole encounters the active         wedge-shaped member it acts as a piston, sliding on the passive         wedge-shaped member so that both wedge-shaped members exert         diametrically opposed forces against a wall of the blast hole         and are locked in place.

In one embodiment the active wedge-shaped member is coated with a friction-reducing surface treatment. The friction-reducing surface treatment may be PTFE paint, liquid or solid friction modifier. Alternatively the active wedge-shaped member may be enclosed within a friction-reducing envelope such as a plastic sleeve, in which the envelope provides a lower coefficient of friction than the surface of the active wedge-shaped member by itself.

Preferably the passive wedge-shaped member is of greater mass than the active wedge-shaped member wherein, in use, the passive wedge-shaped member provides greater resistance to movement than the active wedge-shaped member. Preferably the passive wedge-shaped member has a friction-increasing material provided on at least part of its surface where it contacts the wall of the blast hole to increase the sliding resistance of the passive wedge-shaped member relative to the active wedge-shaped member.

Preferably the wedge-shaped members are formed from a single cylindrical elongate member cut through at an angle to form the pair of wedge-shaped members. Preferably each end of the cylindrical elongate member is substantially flat, having a surface that is substantially orthogonal to a longitudinal axis of the cylindrical elongate member. Preferably the cylindrical elongate member is cut through at an acute angle of between about 55° and 85°. More preferably the cylindrical elongate member is cut through at an acute angle of between about 75° and 85°.

Preferably the cylindrical elongate member forms a solid core of the plug. Preferably the solid core is about 250 mm to 600 mm in length.

Preferably the stemming plug further comprises an outer liner having a first inner join line and a second outer join line, the inner join line being made of water-soluble material and defining a first diameter of the liner which is smaller than a second diameter defined by the second join line and is adapted to be received in the blast hole. Typically the liner is made from a water-absorbent material, with suitable friction-modifying properties to provide grip in the blast hole during installation.

According to another aspect of the present invention there is provided a stemming plug for stemming a blast hole in a mine, the plug comprising:

-   -   an outer liner having a first inner join line and a second outer         join line, the inner join line being made of water-soluble         material and defining a first diameter of the liner which is         smaller than a second diameter defined by the second join line         and is adapted to be received in a blast hole; and,     -   a pair of elongate wedge-shaped members received in sliding         relationship with respect to each other within the outer liner,         one of the wedge-shaped members having a friction reducing         material provided on at least part of its surface to reduce the         sliding resistance of the one wedge-shaped member relative to         the other wedge-shaped member;     -   wherein, in use, when the plug is immersed in water it dissolves         the first inner join line, and when the plug is tamped into the         blast hole the sleeve expands to the second diameter and the         wedge-shaped members slide relative to each other so as to wedge         into the blast hole and block the blast hole.

According to a further aspect of the present invention there is provided a stemming plug for stemming a blast hole in a mine, the plug comprising:

-   -   an outer liner adapted to be received in a blast hole; and,     -   a pair of elongate wedge-shaped members received in sliding         relationship with respect to each other within the liner,         wherein one of the wedge-shaped members has less mass than the         other wedge-shaped member so that under dynamic loading the         lighter wedge-shaped member will accelerate faster than the         other, heavier wedge-shaped member; and,     -   wherein, in use, when the plug encounters a shockwave after         initiation of an explosive in the blast hole the lighter         wedge-shaped member moves relative to the heavier wedge-shaped         member so as to wedge the pair of wedge-shaped members into the         blast hole and block the blast hole.

Preferably the one wedge-shaped member is an active wedge-shaped member having a sloping face received in sliding relationship with a matching face of the other wedge-shaped member, the other wedge-shaped member being a passive wedge-shaped member wherein, in use, the active wedge-shaped member is positioned nearest to an explosive material in the blast hole and the passive wedge-shaped member is positioned further from the explosive material in the blast hole.

In one embodiment the active wedge-shaped member is coated with a friction-reducing surface treatment. The friction-reducing surface treatment may be PTFE paint, liquid or solid friction modifier. Alternatively the active wedge-shaped member may be enclosed within a friction-reducing envelope, such as a plastic sleeve, in which the envelope has a lower coefficient of friction than the surface of the active wedge-shaped member by itself.

In another embodiment the stemming plug further comprises an installation wedge-shaped member provided at a base of the stemming plug, and wherein the passive wedge-shaped member also has a wedge-shaped base whereby, in use, the installation wedge-shaped member is forced against the passive wedge-shaped member during installation.

Preferably the wedge-shaped members are formed from a single cylindrical elongate member cut through at an angle to form two or more wedge-shaped members.

According to a still further aspect of the present invention there is provided a method of stemming a blast hole in a mine, the method comprising:

-   -   filling an outer liner with a pair of elongate wedge-shaped         members received in sliding relationship with respect to each         other to form a stemming plug, the liner having a first inner         join line and a second outer join line, the inner join line         being made of water-soluble material and defining a first         diameter of the liner which is smaller than a second diameter         defined by the second join line and is adapted to be received in         a blast hole;     -   providing a friction reducing material on at least part of the         surface of one of the wedge-shaped members, to reduce the         sliding resistance of the one wedge-shaped member relative to         the other wedge-shaped member;     -   providing one or more of the stemming plugs to a mine site ready         for use in the event that a blast hole needs to be blocked;     -   immersing one or more of the stemming plugs in water for a         prescribed time until the water dissolves the first inner join         line;     -   inserting the one or more wetted stemming plugs in the blast         hole; and,     -   jolting each stemming plug so that the liner expands to the         second diameter and the wedge-shaped members slide relative to         each other so as to wedge into the blast hole and block the         blast hole.

Preferably the method further comprises the step of tamping the stemming plug to firmly locate the wedge-shaped members in the blast hole.

Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Likewise the word “preferably” or variations such as “preferred”, will be understood to imply that a stated integer or group of integers is desirable but not essential to the working of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature of the invention will be better understood from the following detailed description of several specific embodiments of a wedge stemming plug and method of stemming a mining blast hole using wedges, given by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a cross-section of a stope in a metalliferous underground mine, showing the preferred location of a wedge stemming plug in a blast hole for preventing the explosives from rifling out of the blast hole;

FIG. 2 is a graph showing the increase in force absorbed by the wedge-shaped members of the stemming plug on a wall of the blast hole with increasing displacement;

FIG. 3 illustrates a first embodiment of a wedge stemming plug according to the present invention;

FIG. 4 illustrates a second embodiment of a wedge stemming plug according to the present invention;

FIG. 5 illustrates a third embodiment of a wedge stemming plug according to the present invention;

FIG. 6 is a cross-sectional view of a fourth embodiment of a wedge stemming plug according to the present invention;

FIG. 7 is a perspective view of the fourth embodiment of the wedge stemming plug shown in FIG. 6;

FIG. 8 shows the wedge-shaped members of FIG. 7 within a partially cut-away elongate sleeve; and,

FIG. 9 is a perspective view of a fifth embodiment of a wedge stemming plug according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A first embodiment of a stemming plug 10 in accordance with the invention, for stemming a blast hole in a mine, is illustrated in FIG. 3. The plug 10 comprises an active wedge-shaped member 12 having a sloping face received in sliding relationship with a matching face of a passive wedge-shaped member 14. In use, the active wedge-shaped member 12 is positioned nearest to an explosive material in the blast hole, and the passive wedge-shaped member 14 is positioned further from the explosive material in the blast hole.

FIG. 1 shows a cross section of a stope in a metalliferous underground mine, with an upwardly extending, vertical blast hole 16. The stemming plug 10 is preferably located near the lower end of the blast hole 16, below explosive material 18, for effectiveness in preventing the explosives from rifling out of the blast hole. Stemming contains the power of the explosive blast to the location where it is needed (indicated by the arrows in FIG. 1).

More effective use of wedging type arrangements for stemming and otherwise blocking drill holes can be made by reducing the friction on the active wedge-shaped member, increasing the friction on the passive wedge-shaped member, or a combination of both.

In the stemming plug 10, the active wedge-shaped member 12 has a friction reducing material provided on at least part of its surface to reduce the sliding resistance of the active wedge-shaped member 12 relative to the passive wedge-shaped member 14. Preferably the active wedge-shaped member 10 is coated with a friction-reducing surface treatment 20. The friction-reducing surface treatment may be PTFE paint, liquid or solid friction modifier.

In use, when a shock wave from initiation of the explosive material in the blast hole encounters the active wedge-shaped member it acts as a piston, sliding on the passive wedge-shaped member so that both wedge-shaped members exert diametrically opposed forces against a wall of the blast hole and are locked in place.

In this arrangement the passive wedge-shaped member 14 is further from the explosive material 18, and the next to encounter the shockwave. The passive wedge-shaped member 14 is preferably relatively stationary, or able to offer greater resistance to movement, in comparison to the active wedge-shaped member 12 for the pair to be effective. Increasing the resistance to movement of the passive wedge-shaped member is achieved in PCT/AU2013/000489 by backing that wedge up with a parcel of grout. The grout increases the resistance to movement of the passive wedge-shaped member through cohesion between the grout and the wall of the blast hole.

However there are other ways that the resistance to movement of the passive wedge-shaped member can be increased. One is by applying a high friction material, such as a surface coating (for example, a rubber coating), to the contact area between the passive wedge-shaped member and the wall of the hole. In FIG. 3, the passive wedge-shaped member 14 has a high friction material 22 applied to a surface thereof where the passive wedge-shaped member 14 contacts the wall of the blast hole.

A further improvement in stemming effectiveness may be achieved by increasing the mass of the passive wedge-shaped member relative to the active wedge-shaped member, so that it offers greater resistance to movement. It can be considered to have a greater stationary inertia. When the combined wedges encounter the blasting force, the active wedge is lighter and will therefore accelerate faster than the larger passive wedge. If these circumstances prevail, the wedges will exert diametrically opposite forces against the wall of the hole and lock in place, providing a maximum amount of stemming resistance to the explosives.

FIG. 4 shows a second embodiment of the stemming plug 30 where the passive wedge-shaped member 34 is longer than the active wedge-shaped member 32 so as to provide increased surface area in contact with the wall of the blast hole. Lengthening the passive wedge-shaped member 34 has the additional advantage of increasing the mass of the passive wedge-shaped member. As with the previous embodiment, the active wedge-shaped member 32 has a friction-reducing coating 20 on its surfaces. In this embodiment, the passive wedge-shaped member 34 has greater resistance to movement due to the greater surface area which is in contact with the wall of the hole.

Also, in this instance, because the passive wedge-shaped member 34 has greater mass, when the blasting force is applied to the combined wedge-shaped members, the active wedge-shaped member 32, (with less mass), will accelerate faster than the passive wedge-shaped member 34. In this way the active wedge-shaped member 32 will apply a greater force to the passive wedge-shaped member 34, and lock against the wall of the hole harder, resisting the blasting forces to a greater extent.

It will be apparent that the maximum advantage is to be gained by:

-   -   Reducing the friction on the active wedge;     -   Increasing the friction on the passive wedge; and,     -   Having a heavier passive wedge-shaped member than active         wedge-shaped member.

FIG. 5 shows a third embodiment of the stemming plug 40 which is similar to the second embodiment 30, and therefore the same reference numerals will be used to identify the like parts. As with the previous embodiment, the passive wedge-shaped member 34 is of greater length, (and therefore has greater mass) than the active wedge-shaped member 32. Furthermore the active wedge-shaped member 32 has a friction-reducing surface treatment 20 applied to its surfaces. In this instance, the passive wedge-shaped member 34 also has a high friction material 22 applied to a surface thereof where the passive wedge-shaped member 34 contacts the wall of the blast hole.

Because there is a difference in the relative frictional resistance of the active wedge-shaped member and passive wedge-shaped member, with the active wedge-shaped member having the lower frictional resistance, under a dynamic force the active wedge-shaped member pistons into the passive wedge-shaped member further increasing the friction between the passive wedge-shaped member and the wall of the hole.

This interaction compounds through the resistance being transferred from the passive wedge to the active wedge. The greater the force applied to the active wedge, causing displacement of the active wedge, the greater the resistance provided by the passive wedge, as shown in FIG. 2. This further increases the frictional resistance between the passive wedge-shaped member and the wall of the hole. This cycle continues until the shockwave from the blast passes or the wedges fail under maximum load. In either case, the wedges have provided the maximum amount of stemming to the blast hole.

FIGS. 6 and 7 show a cross-sectional view and a perspective view respectively of a fourth embodiment of the wedge stemming plug 50, which is similar to the first embodiment illustrated in FIG. 3 and therefore the same reference numerals will be used to identify the like parts. In this embodiment, the active wedge-shaped member 12 is enclosed within a friction-reducing envelope, such as a plastic sleeve 52, in which the envelope provides a lower coefficient of friction than the surface of the active wedge-shaped member by itself. When the active wedge-shaped member 12 has the blast force acting upon it, it is forced against the passive wedge-shaped member 14. The plastic sleeve 52 is long enough to allow for the sliding movement of the active wedge-shaped member 12. The arrow in FIG. 6 shows the direction of movement.

Both the active and passive wedge-shaped members have a matching, mating sliding surface at an acute angle to the direction of force from the initiation of the explosive material. Ideally the wedge-shaped members are cut from the same core material so that the sliding surface is identical on each wedge. Preferably both of these faces are treated with a friction reducing coating or arrangement, so that the sliding surfaces move freely. In the event that the active wedge-shaped member is enveloped in a plastic sleeve, this reduction in friction also applies to the passive wedge at this sliding face even though it is not within the plastic sleeve.

Preferably the wedge stemming plug further comprises an outer liner 56, the wedge-shaped members being received within the outer liner. The liner 56 is similar to the elongate sleeve of porous material described in PCT/AU2013/000489. The outer liner 56 preferably has a first inner join line and a second outer join line, the inner join line being made of water-soluble material and defining a first diameter of the liner which is smaller than a second diameter defined by the second join line and is adapted to be received in the blast hole.

FIG. 8 shows the wedge-shaped members of FIG. 7 within a partially cut-away outer liner 56. Typically the liner 56 is made from a water-absorbent material, with suitable friction properties to provide grip in the blast hole during installation. Preferably the liner 56 is made from a lightweight, biodegradable material. Advantageously the liner 56 is made from hessian or jute, which is a low cost, environmentally sustainable material.

The passive wedge-shaped member 14 is provided with a high friction material 22 applied thereto, such as a rubber compound. Advantageously the rubber compound 22 is applied so that is permeates the outer liner 56 along the surface of the passive wedge-shaped member 14 where it contacts the wall of the hole, so that the compound 22 contacts both the passive wedge-shaped member 14 and the wall of the hole in use.

Preferably the wedge-shaped members are formed from a single cylindrical elongate member cut through at an angle to form the pair of wedge-shaped members, similar to the wedge-shaped members described in PCT/AU2013/000489. Preferably each end of the cylindrical elongate member is substantially flat, having a surface that is substantially orthogonal to a longitudinal axis of the cylindrical elongate member. Preferably the cylindrical elongate member is cut through at an acute angle of between about 55° and 85°. More preferably the cylindrical elongate member is cut through at an acute angle of between about 75° and 85°.

Preferably the cylindrical elongate member forms a solid core of the plug. Typically the solid core has an outer diameter about 10% less than the nominal drill bit size to allow room for bit wear, and a further 3 mm smaller to allow for a signal tube. Preferably the solid core is about 250 mm to 600 mm in length. Typically the solid core of the plug is formed from cured grout material. Preferably the solid core is a grout plug made of general purpose (Portland) cement reinforced with fibres for additional strength and toughness. Typically the reinforcing fibres are either one or a combination of 47 mm monofilament poly fibres and 19 mm monofilament fibres, of the kind manufactured by Radmix and typically used in shotcrete applications.

A further possible configuration using the same philosophy is to have an additional installation wedge at the base of the stemming plug. In this configuration, a central passive wedge remains significantly longer and heavier than either the active wedge or the installation wedge. Simple geometry and being made of the same materials means it as a minimum is approximately twice the mass of the active wedge or the installation wedge.

FIG. 9 illustrates a fifth embodiment of the wedge stemming plug 60, which is similar to the previous embodiment illustrated in FIG. 8 and therefore the same reference numerals will be used to identify the like parts and these will not be described again in detail. The principal difference in this embodiment is the addition of an installation wedge-shaped member 62 at the base of the stemming plug 60. A central wedge-shaped member 64 is the passive wedge-shaped member, and is approximately twice the mass of the active wedge-shaped member 12. Instead of having a flat-faced base, as with the previous passive wedge-shaped members 14 and 34, the central passive wedge-shaped member 64 also has a wedge-shaped base.

The advantage of such a configuration is that it allows ease of installation, whereby, in use, the installation wedge-shaped member 62 is forced against the central passive wedge-shaped member 64 during installation. It has the advantage of being jolted against the passive wedge-shaped member 64, typically of twice its own mass, allowing it to be more easily placed in steeply angled upholes. However the wedge stemming plug 60 can also be placed in flat holes where gravity does not assist the locking of the wedges.

A further advantage is that the plug 60 is less likely to rattle loose in the blast hole if there is significant vibration from other holes being initiated around it. An even further advantage is that the active wedge-shaped member 12 in this instance is not used to locate the plug, so it is freer to move in a pistoning manner with the force of the initiating explosive material. Additionally, if the wedge-shaped members are located relative to each other as in FIG. 9, on location in the blast hole the passive wedge-shaped member 64 is already in a position to receive the active wedge-shaped member 12 on initiation. That is, the central passive wedge-shaped member 64 is already forced against the wall of the blast hole and has frictional resistance ready to receive the pistoning active wedge-shaped member 12.

The angle of the sliding surfaces between the installation wedge-shaped member 62 and the passive wedge-shaped member 64 need not be exactly the same as that between the passive wedge-shaped member and the active wedge-shaped member, but for all intents and purposes, it is similar.

Since the installation wedge-shaped member 62 is used primarily for initially locating the plug, it is required to grip the hole. In this instance, the installation wedge-shaped member 62 is free of friction-modifying materials except for on the sliding surface which mates with the passive wedge-shaped member 64. On this surface, and the matching surface on the passive wedge-shaped member, friction-reducing materials would be advantageous. In the instance of the plastic sleeve 52 being used in the plug 60, the plastic sleeve 52 would not encompass the installation wedge-shaped member 62, but could go between the installation wedge-shaped member 62 and the central passive wedge-shaped member 64, on the matching, sliding surface (as shown in FIG. 9).

Now that preferred embodiments of the wedge stemming plug and a method of stemming a hole have with wedges have been described in detail, it will be apparent that the described embodiments provide a number of advantages over the prior art, including the following:

-   -   (i) The plugs are more effective than other uphole stemming         products at containing the forces produced upon detonation of         the explosive material.     -   (ii) They are cost effective being made of low cost materials.     -   (iii) They are very quick and easy to install.     -   (iv) No special installation tools are required.     -   (v) Modifying the frictional properties of the wedge-shaped         members, and providing a difference in mass between the         wedge-shaped members for different relative resistance         characteristics, can improve the pistoning effect where the         wedge nearest the blast is forced into the adjacent wedge         further from the blast.     -   (vi) The plug configuration also ensures the upper wedge-shaped         member transmits the larger proportion of the shockwave force         into resistance against the blast by the wedging action.     -   (vii) A further, significant advantage of this configuration is         that the wedge-shaped members can be made very economically and         simply from a previously cured grout plug.

It will be readily apparent to persons skilled in the relevant arts that various modifications and improvements may be made to the foregoing embodiments, in addition to those already described, without departing from the basic inventive concepts of the present invention. For example, whilst hessian or jute has been described as the preferred material for making the outer liner, the outer liner may be made from any suitable material. Therefore, it will be appreciated that the scope of the invention is not limited to the specific embodiments described and is to be determined from the appended claims. 

1. A stemming plug for stemming a blast hole in a mine, the plug comprising: an active wedge-shaped member having a sloping face received in sliding relationship with a matching face of a passive wedge-shaped member wherein, in use, the active wedge-shaped member is positioned nearest to an explosive material in the blast hole and the passive wedge-shaped member is positioned further from the explosive material in the blast hole; and, the active wedge-shaped member having a friction reducing material provided on at least part of its surface to reduce the sliding resistance of the active wedge-shaped member relative to the passive wedge-shaped member; whereby, in use, when a shockwave from initiation of the explosive material in the blast hole encounters the active wedge-shaped member it acts as a piston, sliding on the passive wedge-shaped member so that both wedge-shaped members exert diametrically opposed forces against a wall of the blast hole and are locked in place.
 2. A stemming plug as defined in claim 1, wherein the active wedge-shaped member is coated with a friction-reducing surface treatment.
 3. A stemming plug as defined in claim 2, wherein the friction-reducing surface treatment is a PTFE paint, liquid or solid friction modifier.
 4. A stemming plug as defined in claim 2, wherein the active wedge-shaped member is enclosed within a friction-reducing envelope such as a plastic sleeve, in which the envelope provides a lower coefficient of friction than the surface of the active wedge-shaped member by itself.
 5. A stemming plug as defined in any one of claims 1 to 4, wherein the passive wedge-shaped member is of greater mass than the active wedge-shaped member wherein, in use, the passive wedge-shaped member provides greater resistance to movement than the active wedge-shaped member.
 6. A stemming plug as defined in any one of claims 1 to 5, wherein the passive wedge-shaped member has a friction-increasing material provided on at least part of its surface where it contacts the wall of the blast hole to increase the sliding resistance of the passive wedge-shaped member relative to the active wedge-shaped member.
 7. A stemming plug as defined in claim 1, wherein the wedge-shaped members are formed from a single cylindrical elongate member cut through at an angle to form the pair of wedge-shaped members.
 8. A stemming plug as defined in claim 7, wherein each end of the cylindrical elongate member is substantially flat, having a surface that is substantially orthogonal to a longitudinal axis of the cylindrical elongate member.
 9. A stemming plug as defined in claim 8, wherein the cylindrical elongate member is cut through at an acute angle of between about 55° and 85°.
 10. A stemming plug as defined in claim 9, wherein the cylindrical elongate member is cut through at an acute angle of between about 75° and 85°.
 11. A stemming plug as defined in claim 7, wherein the cylindrical elongate member forms a solid core of the plug.
 12. A stemming plug as defined in claim 11, wherein the solid core is about 250 mm to 600 mm in length.
 13. A stemming plug as defined in any one of claims 1 to 12, wherein the stemming plug further comprises an outer liner having a first inner join line and a second outer join line, the inner join line being made of water-soluble material and defining a first diameter of the liner which is smaller than a second diameter defined by the second join line and is adapted to be received in the blast hole.
 14. A stemming plug as defined in claim 13, wherein the liner is made from a water-absorbent material, with suitable friction properties to provide grip in the blast hole during installation.
 15. A stemming plug for stemming a blast hole in a mine, the plug comprising: an outer liner having a first inner join line and a second outer join line, the inner join line being made of water-soluble material and defining a first diameter of the liner which is smaller than a second diameter defined by the second join line and is adapted to be received in a blast hole; and, a pair of elongate wedge-shaped members received in sliding relationship with respect to each other within the liner, one of the wedge-shaped members having a friction reducing material provided on at least part of its surface to reduce the sliding resistance of the one wedge-shaped member relative to the other wedge-shaped member; wherein, in use, when the plug is immersed in water it dissolves the first inner join line, and when the plug is tamped into the blast hole the liner expands to the second diameter and the wedge-shaped members slide relative to each other so as to wedge into the blast hole and block the blast hole.
 16. A stemming plug for stemming a blast hole in a mine, the plug comprising: an outer liner adapted to be received in a blast hole; and, a pair of elongate wedge-shaped members received in sliding relationship with respect to each other within the liner, wherein one of the wedge-shaped members has less mass than the other wedge-shaped member so that under dynamic loading the lighter wedge-shaped will accelerate faster than the other, heavier wedge-shaped member; and, wherein, in use, when the plug encounters a shockwave after initiation of an explosive in the blast hole the lighter wedge-shaped member moves relative to the heavier wedge-shaped member so as to wedge the pair of wedge-shaped members into the blast hole and block the blast hole.
 17. A stemming plug as defined in claim 15 or in claim 16, wherein the one wedge-shaped member is an active wedge-shaped member having a sloping face received in sliding relationship with a matching face of the other wedge-shaped member, the other wedge-shaped member being a passive wedge-shaped member wherein, in use, the active wedge-shaped member is positioned nearest to an explosive material in the blast hole and the passive wedge-shaped member is positioned further from the explosive material in the blast hole.
 18. A stemming plug as defined in claim 17, wherein the active wedge-shaped member is coated with a friction-reducing surface treatment.
 19. A stemming plug as defined in claim 18, wherein the friction-reducing surface treatment may be PTFE paint, liquid or solid friction modifier.
 20. A stemming plug as defined in claim 18, wherein the active wedge-shaped member is enclosed within a friction-reducing envelope, such as a plastic sleeve, in which the envelope has a lower coefficient of friction than the surface of the active wedge-shaped member by itself.
 21. A stemming plug as defined in any one of claims 17 to 20, wherein the stemming plug further comprises an installation wedge-shaped member provided at a base of the stemming plug, and wherein the passive wedge-shaped member also has a wedge-shaped base whereby, in use, the installation wedge-shaped member is forced against the passive wedge-shaped member during installation.
 22. A stemming plug as defined in claim 21, wherein the wedge-shaped members are formed from a single cylindrical elongate member cut through at an angle to form two or more wedge-shaped members.
 23. A method of stemming a blast hole in a mine, the method comprising: filling an outer liner with a pair of elongate wedge-shaped members received in sliding relationship with respect to each other to form a stemming plug, the liner having a first inner join line and a second outer join line, the inner join line being made of water-soluble material and defining a first diameter of the liner which is smaller than a second diameter defined by the second join line and is adapted to be received in a blast hole; providing a friction reducing material on at least part of the surface of one of the wedge-shaped members, to reduce the sliding resistance of the one wedge-shaped member relative to the other wedge-shaped member; providing one or more of the stemming plugs to a mine site ready for use in the event that a blast hole needs to be blocked; immersing one or more of the stemming plugs in water for a prescribed time until the water dissolves the first inner join line; inserting the one or more wetted stemming plugs in the blast hole; and, jolting each stemming plug so that the liner expands to the second diameter and the wedge-shaped members slide relative to each other so as to wedge into the blast hole and block the blast hole.
 24. A method of stemming a blast hole in a mine as defined in claim 23, wherein the method further comprises the step of tamping the stemming plug to firmly locate the wedge-shaped members in the blast hole. 